1. Field of the Invention
The present invention relates to data storage devices that employ aerodynamically supported transducing sliders for reading and recording magnetic data, and more particularly to the prediction of failure of heating devices resident within the transducing sliders.
2. Description of the Related Art
A magnetic direct access storage device or disk drive contains at least one rotating disk covered with a magnetic coating which can store electronic data, and an apparatus for reading data from and writing data to that disk. This is implemented by a motor to rotate the disk, a transducer to read and write data to and from the disk, an actuator arm to position the transducer over the disk surface, and associated electronic circuitry to transfer information between the transducer and an accessing source. A typical configuration is shown in FIG. 1 (Prior Art). Data stored on the surface of magnetic disk 100 is accessed by transducers contained within a slider 104 mounted on the end of actuator arm 102. The slider 104 is attached to the end of the actuator arm 102 via a load beam or suspension component (not shown). This structure is housed within, for example, a typical hard disk drive (not shown). The slider is so named because disk 100 is rotated at high speeds to create an air cushion or bearing that supports the slider at a controlled distance from its associated recording surface. The slider contacts the disk surface only when the disk is either stationery, accelerating from a stop, or decelerating to a complete stop. The slider is also referred to as the transducing head or just head, because it contains the read and write magnetic head structures as an integral portion of its construction.
FIG. 2 (Prior Art) and FIG. 3 (Prior Art) are side views of typical slider configurations of the prior art. In these figures, slider 104 has a beveled leading edge at the left, with the read/write transducers mounted in the trailing edge portion 204. Media surface 202 is moving to the right, as indicated by arrow 206, while slider 104 is approximately stationary. The beveled portion of the slider aids in aerodynamically lifting the slider 104 and suspending it above the media surface 202 as disk 100 rapidly spins below arm 102 and slider 104. At current areal recording densities, the “flying height” between the slider and media has been reduced to the sub-micron or nanometer range. The higher the areal recording density, the closer the transducers must be to the surface storing the magnetic information to avoid errors and lost data while reading or writing to the media. To aid in controlling these small dimensions, designers and manufacturers have introduced a thermal mechanism into the slider structure to place the transducer surfaces closer to the media surface, without having to alter the nominal “flying height” of the slider body. This is done by placing localized heating within the slider 104 near the location of the transducers, causing region 204 to preferentially expand. This expansion causes the transducers to be placed even closer to the media surface. Thus, the slider is heated while the heads are placed over the rotating media reading or writing data. The heater is turned off when the heads are in contact with the media during the starting or stopping of the disk. The transducer surface region 204 is recessed above the bottom of the slider when heating is off, which allows the slider to contact the media surface while the disk is starting up or slowing down without frictional wear on the transducer surfaces 204.
FIG. 4 (Prior Art) is a partial cut away view of the transducer containing portion of the unheated slider of FIG. 2. The nominal “flying height” of the slider 104 is indicated by hs. A heater structure 406 is placed proximate to write head structure 402, and is powered by switch 408a and power supply 410. In the unheated state, the transducer surface region 204 is recessed above the bottom of the slider body, such that transducer flying height d (ref 404) is greater than hs. As previously mentioned, this prevents the transducer surfaces from wearing when the slider contacts the disk surface as the media begins or ends rotation. In this figure, the media 100 is moving to the left as indicated by arrow 412.
FIG. 5 (Prior Art) is a partial cut away view of the transducer containing portion of the heated slider of FIG. 3. With heater structure 406 powered via closed switch 408b, the transducer containing region of the slider expands preferentially in comparison with the unheated portion, causing the transducer flying height d (ref 502) to be less than the nominal slider flying height hs. Since maintaining the transducer flying height is critical to providing error free performance of the drive, monitoring the condition of the heater becomes an important requirement for providing a high reliability storage device. Heater failure can be catastrophic for drive operation, because it will result in the transducer flying height 404 to be even greater than the slider flying height hs, due to the cooling of the slider (FIG. 4). At the greater transducer flying height, error rates will increase significantly due to the high areal densities of the stored data.
What is needed is a method for monitoring the performance of the heater, which provides advance warning to the user of eminent heater failure, prior to significant loss of data or increased error rates.
U.S. Pat. No. 6,336,083 discloses a method of predicting failure of resistive element heaters using a compiled database of measured ratiometric factors affecting heater life. The method can either be carried out actively, by continuously measuring known factors affecting heater life and decrementing a count of the remaining heater life, or the method may be carried out passively by estimating the operating profile and the averages within each segment of the profile, of the factors affecting heater life. This method is generally applied to industrial cartridge and coil type heaters, and requires heater element coil temperatures coupled with a complex heat transfer model to predict heater life. This degree of complexity is not useful for the prediction of slider heater lifetimes, due to the difficulty of obtaining detailed heater element temperature data within the slider. The predictive modeling is made more difficult due to the convective cooling experienced by a slider over moving media.
Japanese patents JP5258838 and JP5258839 disclose methods to predict the lifetime of industrial heaters by comparing a measured value of the initial heater resistance with a currently measured value, and based on the comparison, predicting the life of the heater. The life of the heater is defined by the time to reach an open circuit state. This method is unsuitable for application in disk drive slider heaters since the failure warning must be provided well before the “burn out” or open circuit state of the heater is reached.
U.S. Pat. No. 5,959,801 discloses an arrangement for reducing the stiction bond at power-up time between an slider parked on a disk or other magnetic recording medium. A thermally expansive medium such as alumina is included in the slider body thermally adjacent to the write element. At power-up time or when a failure of the disk, etc. to rotate is detected during startup, a current is applied to the write element. The volume expansion resulting from the heat causes a change in the shape and/or location of the slider surface in near contact with the disk and thereby breaks or reduces the slider/disk stiction bond.
U.S. Pat. No. 5,991,113 discloses a device for reading and recording magnetic data including an aerodynamically supported slider with an air bearing surface, and a transducer mounted to the slider for movement toward and away from the air bearing surface responsive to changes in the slider operating temperature. In one embodiment, the transducer movement is primarily due to a difference in thermal expansion coefficients between a transducing region of the slider incorporating the transducer, and the remainder of the slider body. In another embodiment, a strip of thermally expansive material is incorporated into the slider near the transducer to contribute to the displacement by its own expansion. A temperature control circuit, coupled to the strip of thermally expansive material or to a resistance heating element on the slider, employs a variable current source to control the slider temperature and transducer displacement. Nominal slider operating temperatures can be set to achieve a predetermined transducer flying height, to compensate for variations in flying heights among batch fabricated sliders. Optionally, a temperature sensor can be employed to measure the slider operating temperatures and provide a temperature sensitive input to the temperature control circuit.
U.S. Pat. No. 5,172,365 discloses a method and apparatus for predicting the approach of semiconductor laser diode end-of-life from the power vs. current characteristic curve of the diode thereby obviating the need for nonvolatile memory. Current measurements are taken for three power levels and the linear slope of the characteristic curve at the high power level is compared to the linear slope at low power levels. When the comparison exceeds predetermined criteria, a flag is raised.
U.S. Pat. No. 5,557,183 discloses a disk drive having a spindle motor, a disk that is rotated by the spindle motor, and a movable actuator arm that carries a read/write head. The head physically engages a parked, or home, position at the Inner Diameter (ID) of the disk when the spindle motor is not energized and the disk is stationary. The electrical energization that must be applied to the spindle motor in order to breakaway the head from the disk (i.e., the breakaway current), and the energization that is necessary to cause the motor to achieve a stable spinning state (i.e., the spin current) are monitored. Possible future failure of the disk drive is predicted as a function of any changes in these two electrical parameters, as these parameters may change over a period of time; i.e., may change over a number of disk drive stop/start events.
U.S. Pat. No. 6,191,697 discloses that, in many technical applications, situations often arise where it is desirable to know, in real time, if a circuit is functioning. That is, it is desirable to know if, for example, a heater element is operational or blown (open). By sensing the current flowing to the circuit, it is easy to determine if the circuit is operational. However, this only provides information while the circuit is in the process of operating. There must be some real-time connection with the request or demand for power to know if in fact the device is non-operational, or if there is simply no controller request for power. A system (and method) according to the invention includes a real-time, simultaneous current and voltage sensing and arbitration device which provides a single, point of use implementation, and allows for detection and signaling of a fault condition. A circuit breaker may be constructed to incorporate such a system.
U.S. Pat. No. 6,249,890 discloses a method and apparatus for predicting future failure of a disc drive head from degradation in a head readback response characteristics, such as electrical resistance, readback signal amplitude, asymmetry or nonlinearity. During manufacturing, the disc drive determines and store a baseline level for a selected readback response characteristic of the head indicative of head performance as data are read back from a disc. During subsequent data processing use, the disc drive subsequently periodically determines a subsequent level for the readback response characteristic of the head. The possibility of a future failure of the drive is next predicted in relation to a difference between the baseline level and the subsequent level for the readback response characteristic of the head. An indication of the possibility of the future failure is provided to allow a host device to reallocate data stored on the disc before the failure of the head.
U.S. Pat. No. 6,373,647 discloses an MR head self-testing method provided to test for instability in MR heads incorporated with a hard disk drive. A first method is carried out with the disk rotating and includes positioning the MR head over a rotating magnetic storage disk and controlling the MR head to read from erased data fields defined on the disk. This read signal is filtered and conditioned according to preprogrammed filter coefficients contained in an FIR filter to provide an exaggerated read error signal. The exaggerated read error signal is provided to a digital comparator and counter circuit for detecting and counting voltage baseline jumps that exceed preprogrammed positive and/or negative threshold values. The counted positive and negative voltage baseline jumps, which are indicative of MR head instability, are provided to an error diagnostic register for analysis. If the error diagnostic register contains single polarity voltage baseline jumps, the voltage baseline jumps may be caused by thermal asperities on the disk. If the error diagnostic register contains both positive and negative voltage baseline jumps, the voltage baseline jumps may be caused by MR head instability. A second method is carried out with the disk stationary so there is no opportunity for thermal asperities to generate baseline jumps. Possible instabilities can only be of the Barkhausen noise or dielectric breakdown types.
U.S. Patent Application Publication No. 2004/0075942 discloses a head fabricated using photolithography, wherein the head is purposely powered up during a material removal process, such as lapping, so that the head's expansion (that would be formed on being powered up during normal usage in a drive) is planarized. Specifically, the head is energized in a manner identical (or similar) to energization of circuitry in the head during normal operation in a drive, even though fabrication of the head has not yet been completed. When energized, a shape that the head would have during normal operation is replicated (or approximated). Therefore, the head's shape includes a expansion of the pole tip region, although the head is only partially fabricated. Thereafter, a portion of the head in the expansion is partially or completely removed, by lapping while energized. The depth of material removal from the head is monitored e.g. by a controller sensitive to a change in electrical characteristic of a device (such as a resistor) that is normally fabricated during photolithography of the head.
U.S. Patent Application Publication No. 2004/0085670 discloses a method for measuring the height of a magneto-resistive element (MRE) on a padded slider in a disc drive based on a change in the resistance of the MRE. During operation of the disc drive a biasing current is applied to the MRE head. Next a resistance value of the MRE head is calculated by determining the voltage drop across the MRE head. The resistance of the MRE head is dependent upon its temperature. The temperature of the MRE lowers as its proximity to the disc increases, as the cooler disc surface acts as a conduit drawing heat away from the MRE. This measured resistance value is compared to a threshold value. The threshold value is based upon the resistance of the MRPE head at a threshold temperature, which corresponds to a specific distance away from the disc surface. If the measured resistance is lower than the threshold resistance the method outputs an indication to a user that the drive is in danger of failing.
U.S. Patent Application Publication No. 2004/0075940 discloses a head for use in a drive including a heating element capable of generating heat sufficient to cause the head to have a shape that is similar or identical to the shape that the head has when performing an operation (e.g. writing) on a recording medium in the drive. The heating element is activated when the operation is not being performed. Hence, a head generates the same amount (or similar amount) of heat and is therefore at the same temperature (also called “operating temperature”), regardless of whether or not an operation (such as writing) is being performed. Therefore, the head maintains a fixed shape or has a shape that varies minimally, within a predetermined range around the fixed shape, that in turn results in maintaining fly height (distance between the head and the recording medium). The heating element may be implemented to use loss mechanisms inherent in a write transducer, e.g. by providing a center tap to the write transducer. When using a center tapped write transducer, currents in phase with one another are provided to perform a write operation. When not performing the write operation, the same currents are provided, but out of phase.